A solid polymer fuel cell (hereinafter, sometimes referred to as “fuel cell”) can easily be reduced in size and weight, and is expected to be put into practical use as an electric power source of mobile vehicles, such as electric motor vehicles and the like, and small-size co-generation systems and the like.
A fuel cell-purpose solid electrolyte membrane (hereinafter, sometimes referred to as “solid electrolyte membrane”) is generally required to have high proton conductivity and high oxidation resistance. In order to satisfy such a characteristic requirement, fluorine-based electrolyte membranes represented by a perfluorosulfonic acid membrane have been used. However, fluorine-based electrolytes are costly, and have problems of, for example, elution of hydrofluoric acid during long-time use or the like.
As materials alternative to the fluorine-based electrolyte membranes, hydrocarbon-based electrolyte membranes made of so-called engineered plastics, such as polyether sulfone (PES), polyether ether-ketone (PEEK), etc., have been proposed. In comparison with the fluorine-based electrolyte membranes, the hydrocarbon-based electrolyte membranes have advantages of being less costly and being free from the risk of elution of hydrofluoric acid or the like. However, since all the hydrocarbon-based electrolyte membranes are aromatic electrolyte membranes that have benzene rings or the like in the molecules, the hydrocarbon-based electrolyte membranes are low in flexibility and, when used as a solid electrolyte membrane of a fuel cell, has problems of formation of cracks or the like.
In view of these problems, solid electrolyte membranes that do not have a benzene ring or the like have been developed. For example, Japanese Patent No. 3572302 discloses a solid electrolyte membrane formed by using an epoxy compound and a silane compound as raw material components. This solid electrolyte membrane, having no benzene ring in its molecule, is excellent in flexibility, but has problems of being inferior in oxidation resistance and the like since the membrane has, in its molecule, an epoxy component derived from the epoxy component.